Conventional oil and gas drilling techniques utilise drill-bits which are conveyed on individual lengths (usually 30 feet) of drill-pipe and rotated from the surface of the drilling rig floor to produce the necessary rotary cutting action required to drill well bores. Alternatively, the rotary cutting action can be supplied by using a Positive Displacement Motor (PDM) located above the drill-bit and connected to the surface by either coil tubing that is provided in one continuous length, or by more conventional drill-pipe. The PDM produces the rotary action when drilling fluid is pumped through it from the surface. The main advantage of using coil tubing in conjunction with a PDM is that of a decrease in the running-in time of the equipment into the well-bore.
Debris or cuttings are produced from the cutting action, which are transported to the top of the well bore by the drilling fluid. In order to clean the well bore effectively the drilling fluid must be pumped at a high enough flow rate to lift the cuttings to the surface. However, only relatively low volumes of drilling fluid can be pumped through the complete Bottom Hole Assembly (BHA) without a large pressure drop at the surface.
This problem can be alleviated by using nitrogen to clean the well-bore which gives increased hole cleaning capabilities.
However, the use of nitrogen gives rise to a second problem, in that, nitrogen can only be pumped through a PDM motor for very short periods of time without damaging the PDM motor. Hence, the benefits of using nitrogen to clean the well-bore with existing technology are limited.
Traditionally, this first problem is overcome by using an additional tool in conjunction with the motor and drilling/milling assembly, known as a drop-ball circulating sub. This tool is run above the motor and is operated by dropping a ball, from the surface, down the drill-pipe or coil tubing. The ball seats on top of a piston within the tool and pressure is applied to the upper end of the piston and ball. The pressure is increased until shear pins, which are located within the main body of the drop-ball circulating sub, break allowing the piston to move axially downwards within the main body thereby uncovering circulating holes in the main body drilled transverse to the centre-line of the drop-ball circulating sub. These holes allow an increased flow rate to be pumped through the drill-pipe or coil tubing, thus giving a more effective hole cleaning capability.
However, this tool has the disadvantage that once the ball has been dropped to the circulating sub, no further milling or drilling can take place as the fluid path to the PDM has been blocked by the ball. If further milling or drilling is required then the tool must be removed from the well-bore so that the ball can be removed. Also, the length of time that the ball takes to drop down the drill-pipe or coil tubing can be considerable.
The second problem of pumping nitrogen is helped, but not solved, by using a drop-ball circulating sub as the drop-ball does not effect a complete seal on the piston allowing nitrogen to flow through the motor.